The correct determination of the boost pressure and the air mass flow in the intake manifold of an internal combustion engine at the position in the air system upstream of a control flap is of central importance in maintaining exhaust gas regulations. As a rule, an engine control for controlling the internal combustion engine uses these variables, so as to maintain the appropriate exhaust gas norms.
The boost pressure and the air mass flow are usually not measured by a sensor, but have to be calculated by a dynamic model in the engine control unit in real time. These calculations are based on the sensor variables or model variables for the pressure in the intake manifold p22 (downstream from the control flap), that is, between the control flap and the inlet valves of the engine), the temperature of the aspirated air T21 (upstream of the control flap), the air mass flow {dot over (m)}1 upstream of a compressor (such as a turbocharger), the setting of the control flap POS and the stored air mass m21 in the section of the air supply system upstream of control flap 7. The relationship is described by the following equations:
                    p        21            ⁡              (        t        )              =          g      ⁡              (                              V            21                    ,                                    m              21                        ⁡                          (              t              )                                ,                                    T              21                        ⁡                          (              t              )                                      )                                          m          .                2            ⁡              (        t        )              =          f      ⁡              (                                            p              21                        ⁡                          (              t              )                                ,                                    p              22                        ⁡                          (              t              )                                ,                      POS            ⁡                          (              t              )                                ,                                    T              21                        ⁡                          (              t              )                                      )                                ⅆ                              m            21                    ⁡                      (            t            )                                      ⅆ        t              =                                        m            .                    1                ⁡                  (          t          )                    -                                    m            .                    2                ⁡                  (          t          )                    
The functions f( ) and g( ) are model functions which describe the relationship between the physical variables. This differential equation has to be quantized by the working method of the engine control unit. This gives a difference equation of the following structure:p21(tκ)=g(V21,m21)(tκ-1),T21)(tκ)){dot over (m)}2(tκ)=f(p21(tκ),p22(tκ),POS(tκ),T21(tκ))m21(tκ)=m21(tκ-1)+Δt·({dot over (m)}1(tκ)−{dot over (m)}2(tκ))tκ=κ·Δt This difference equation as algorithm for the solution of the above differential equation is obtained using the so-called explicit Euler method. The use of an explicit method has the following disadvantages particularly for the quantization of air system variables:                In certain operating ranges, this algorithm has a dynamic inaccuracy which could, under certain circumstances, also lead to instability. These dynamic inaccuracies or instabilities depend on the control flap setting and on the volume of the air system section upstream of the control flap.        The calculation has to be carried out, for this reason, using very small time steps, in order to achieve a meaningful stability range. This considerably increases the computational time requirements and ties down considerable computing capacity of the engine control unit.        The above model of the difference equations calculates a stationary pressure drop even at a fully opened control flap, which does not correspond to reality, as a rule. This leads to calculating inaccuracies, and thereby to an inaccurate determination of the boost pressure.        The temperature of the aspirated outside air is recorded digitally. If its value quantization has no sufficient solution, the inverting of the least significant bit of the digital temperature signal may lead to a noise-infested air mass signal {dot over (m)}2. Consequently, an additional filtering of air mass signal {dot over (m)}2 is required. This filtering impairs the achievable dynamics, so that the latter cannot be completely utilized.        
It is therefore an object of the present invention to provide a method and a device for the improved real time capability determination of an air system variable, particularly of the boost pressure and/or of the air mass flow in the air supply system, which avoid the abovementioned problems.